An important development in natural gas production in recent decades, at least in the continental United States, has been the improvement of hydraulic fracturing techniques for stimulating production from previously uneconomically tight formations. For example, the largest gas field put on production in the lower forty eight states in the last twenty years is the Bob West Field in Zapata County, Tex. This field was discovered in the 1950's but was uneconomic using the fracturing techniques of the time where typical frac jobs injected 5,000-20,000 pounds of proppant into a well. It was not until the 1980's that large frac jobs became feasible where in excess of 300,000 pounds of proppant were routinely injected into wells. The production from wells in the Bob West Field increased from a few hundred MCF per day to thousands of MCF per day. Without the development of high volume frac treatments, there would be very little deep gas produced in the continental United States.
The fracing of deep, high pressure gas zones has continued to develop or evolve. More recently, multiple gas bearing zones encountered in deep vertical wells are fraced one after another. This is accomplished by perforating and then fracing a lower zone, placing a bridge plug in the casing immediately above the fraced lower zone thereby isolating the fraced lower zone and allowing a higher zone to be perforated and fraced. This process is repeated until all of the desired zones have been fraced. Then, the bridge plugs between adjacent zones are drilled out and gas from the fraced zones produced in a commingled stream. The result is a well with a very high production rate and thus a very rapid payout.
Another situation where multizone fracing has created commercial wells from previously non-commercial zones is in relatively shallow, moderately pressured tight gas bearing sands and shales, of which the Barnett Shale west of Fort Worth, Tex., is a leading example. By fracing multiple zones of the Barnett Shale, commercial wells are routinely made where, in the past, only non-economic production was obtained.
It is no exaggeration to say that the future of gas production in the continental United States is from heretofore uneconomically tight gas bearing formations. Accordingly, a development that allows effective frac jobs at overall lower costs is important.
Disclosures of interest relative to this invention are found in U.S. Pat. Nos. 2,368,428; 3,289,762; 4,427,071; 4,444,266; 4,637,468; 4,813,481; 5,012,867; 6,227,299; 6,575,249 and 6,732,803.